The present invention relates to a color cathode ray and particularly to a color cathode ray tube which restricts a landing displacement of electron beams on a phosphor layer caused by thermal expansion of a shadow mask.
In general, a color cathode ray tube comprises a vacuum envelope, which includes a face panel having a substantially rectangular effective portion in form of a curved surface, and a funnel connected with the face panel. A phosphor screen made of a three-color phosphor layer which radiates in blue, green, and red is formed on the effective portion of the face panel. A shadow mask is arranged inside the phosphor screen with a predetermined distance maintained from the face panel. The shadow mask comprises a substantially rectangular mask body and a substantially rectangular mask frame equipped at a peripheral portion of the mask body.
The mask body comprises a main surface portion having a number of electron beam apertures formed in a predetermined array and made of a curved surface opposed to the phosphor screen, a non-aperture portion surrounding the main surface portion, and a skirt portion provided around the main surface portion with the non-aperture portion interposed therebetween. The mask frame is formed to have a L-shaped cross-section and is welded to the skirt portion of the mask body.
Meanwhile, an electron gun which emits three electron beams is provided in the neck of the funnel. The three electron beams emitted from the electron gun are deflected by a magnetic field generated by a deflector equipped outside the funnel so as to scan horizontally and vertically the phosphor screen, thereby forming a color image.
In a color cathode ray tubes constructed in a structure as described above, and particularly, in an inline type color cathode ray tube having an electron gun which emits three electron beams arranged in line and running on one same horizontal plane, the three-color phosphor layers are formed of strip-like layers elongated in the vertical direction (or short axis direction or Y-axis direction) perpendicular to the tube axis (or Z-axis). On the other hand, electron beam apertures are arranged such that rows each consisting of a plurality of apertures aligned in the vertical direction and the rows are disposed in the horizontal direction (or long axis direction or X-axis direction).
The shadow mask is provided to select three electron beams, which pass through beam apertures at different angles respectively, so that the electron beams land on predetermined phosphor layers. Further, in order to obtain excellent color purity of an image displayed on the phosphor screen by scanning by respective electron beams, three electron beams passing through the electron beam apertures must correctly land on predetermined phosphor layers, respectively. The mask body therefore must be correctly positioned and aligned in a predetermined relationship to the phosphor screen, and the relationship must be maintained during operation of the color cathode ray tube. In particular, the distance (or q-value) between the inner surface of the effective portion of the face panel and the main surface portion of the mask body must be maintained within a predetermined tolerable range.
However, from operational principles of a color cathode ray tube, those electron beams that pass through electron beam apertures of the mask body and reach the phosphor screen are ⅓ in amount of the entire electron beams emitted from the electron gun, and most of the rest of the electron beams collide with the mask body and are converted into thermal energy, thereby heating the mask body to about 80xc2x0 C. Therefore, the surface portion of the mask body locally expands toward the phosphor screen due to thermal expansion, i.e., so-called doming occurs, particularly in case of a shadow mask whose mask body is mode of a cold-rolled plate having a large thermal expansion coefficient (1.2xc3x9710xe2x88x926/xc2x0 C.) and thickness of 0.1 to 0.3 mm, and whose mask frame is made of a cold-rolled plate having a thickness of about 1 mm and having a greater mechanical strength than the mask body. Consequently, the distance between the inner surface of the effective portion and the main surface of the mask body exceeds a tolerable value, and landing of electron beams onto the three-color phosphor layers is displaced thereby deteriorating color purity.
There are two types of landing drift of electron beams on the three-color phosphor layers, one being landing drift which occurs due to thermal expansion of the entire mask body in the initial period when the color cathode ray tube is started operating, and the other being landing drift due to localized doming which occurs when a high-luminance image is displayed locally. The amount of landing drift differs depending on the luminance of an image pattern displayed on the screen, the duration thereof, and the like. For example, when a high-luminance image is displayed on the entire screen, deterioration of color purity occurs over a large area of the screen. When a high-luminance image is displayed locally, localized doming of the shadow mask occurs and landing positions are greatly drifted in a short time period, resulting in localized deterioration of color purity.
Landing drift due to localized doming is the greatest at an elliptic area in a middle portion of the phosphor screen in the horizontal direction when a high-luminance pattern is displayed at a position which is distant from the center of the screen by about ⅓ W where the length of the phosphor screen in the horizontal direction is expressed as W.
Conventionally, several measures have been developed to restrict landing drift caused by doming of the mask body. For example, the following (a) and (b) are known as techniques for restricting landing drift in the initial period of staring operation of a color cathode ray tube.
(a) According to the technique disclosed in U.S. Pat. No. 2,826,538, a graphite layer containing graphite as a main component is provided on the surface of a main surface a mask body and is used as a radiator for decreasing the temperature of the mask body, in order to promote thermal radiation of a mask body.
(b) Japanese Patent Application KOKAI Publication 60-54139 discloses a mask body in which a glass layer made of lead-borate glass or the like is formed on the surface of a main surface portion of the mask body facing an electron gun. If a lead-borate glass layer is thus provided, less calories are transmitted to the mask body since the thermal conductivity of the layer is smaller than that of the mask body, and therefore, an increase of the temperature of the mask body can be restricted. In addition, by providing a lead-borate glass layer, the mechanical strength of the mask body is improved. Further, if the lead-borate glass is welded to the mask body and crystallized, a compressive stress acts on the glass layer and a tensile stress acts on the mask body, so that the tensile strength of the mask body is improved.
It is also possible to restrict localized doming of the mask body by the techniques as described above.
In addition, the following method (c) is known as a conventional measure for restricting localized doming of the mask body.
(c) The method is to increase the curvature of the mask body. As is known, it is effective for this method to increase the curvature of the mask body in the short axis thereof.
However, in the technique (a) of providing a graphite layer on the surface of a main surface portion of the mask body, adherence of the graphite layer is deteriorated by a heat treatment repeated in steps of manufacturing a color cathode ray tube, so that the graphite layer easily peels off by a vibration applied to the color cathode ray tube. Small fragments of the layer which peeled off stick to the mask body, thereby clogging electron beam apertures, so that the quality of an image displayed on the phosphor screen is deteriorated. Small fragments of the layer also stick to an electron gun or the vicinity thereof, inducing a spark discharge, so that problems such as a reduction of the withstand voltage characteristic and the like easily occur.
In a method of providing a glass layer made of lead-borate glass or the like on the surface of a main portion of a mask body facing an electron gun as indicated in (b), since a large amount of lead oxide (PbO) is contained in the lead-borate glass, diffused reflection of electron beams shielded by a shadow mask increases in the tube, thereby lowering contrast, normally called whiteout. If a lead-borate glass layer is provided on a mask body made of a cold-rolled plate having a thickness of 0.1 to 0.3 mm, a compressive stress and a tensile stress act on the glass layer due to welding and crystallization. Although a preferable thickness of the glass layer is said to be normally to 10 to 20 xcexcm, there is a problem that the mask body is deformed if a glass layer having a thickness of 20 xcexcm or more is formed due to unevenness of manufacturing precision on a mask body made of a cold-rolled plate having a thickness of 0.2 mm or less, for example.
Also, in case of adopting a technique of enlarging the curvature of a main portion of a mask body as in the method (c) in a recent color cathode ray tube with a flattened face panel having an effective portion of a small curvature, the curvature of the inner surface of an effective portion of a shadow mask is small and the curvature of the main surface portion of the mask body is accordingly small throughout from the center of the mask body to the periphery thereof. Therefore, in a flattened color cathode ray tube, an area where doming easily occurs tends to spread to the periphery of longer edges of the mask body.
Further, in order to enlarge the curvature of the main surface portion of the mask body in a flattened color cathode ray tube, the curvature of the inner surface of the effective portion of the face panel must be enlarged. Therefore, particularly in case of a wide color cathode ray tube whose screen has an aspect ratio of 4:3, the difference in thickness between the center portion and the peripheral portion of the face panel is as large as cannot be preferred in view of characteristics. In a normal color cathode ray tube, the heat capacity differs between a main surface portion of the mask body where electron beam apertures are formed and a non-aperture portion where no electron beam apertures are formed, so that a difference in thermal conductivity appears between the main surface portion and the non-aperture portion. Therefore, the mask body has such a temperature distribution that the main surface portion has a very high temperature in relation to the temperature of the non-aperture portion, resulting in that doming in the main surface portion easily becomes large.
The present invention has been made in view of the above problem, and has an object of providing a color cathode ray tube which is capable of reducing landing drift of electron beams on phosphor layers caused by doming of a shadow mask and is difficult to cause deterioration of color purity.
To achieve the above object, a color cathode ray tube according to the present invention comprises: an envelope including a face panel having an inner surface on which a phosphor screen is formed; a shadow mask provided in the envelope and opposed to the phosphor screen; and an electron gun provided in the envelope, for emitting an electron beam onto the phosphor screen through the shadow mask. The shadow mask includes a mask body in form of a substantially rectangular shape, having a main surface portion opposed to the phosphor screen and having a number of electron beam apertures formed therein, a skirt portion provided around the main surface portion with a non-aperture portion interposed between the main surface portion and the skirt portion, and long and short axes perpendicular to each other, and a mask frame in form of a substantially rectangular shape, equipped on the skirt portion. Further, the skirt portion has a plurality of slit-like openings extended in a direction of the long axis of the mask body or elongated concave portions.
According to the present invention, the non-aperture portion may have a plurality of slit-like openings extended in a direction of the long axis of the mask body or elongated concave portions.
Further, according to the present invention, each of the skirt portion and the non-aperture portion of the mask body has a plurality of slit-like openings extended in a direction of the long axis of the mask body or elongated concave portions.
In the color cathode ray tube constructed in a structure as described above, the openings and the concave portions are formed within a range of about xc2xc of a length of the mask body in the direction of the long axis of the mask body, with respect to a center of the range defined at a position distant from the short axis by about ⅓ of the length of the mask body in the direction of the long axis.
In another color cathode ray tube according to the present invention, at least one of the skirt portion and the non-aperture portion has a plurality of circular openings or concave portions a part of which is formed at a high density, and the part has a rectangular shape.
As has been described above, in the color cathode ray tube according to the present invention, the skirt portion of the mask body has openings or concave portions elongated in the long axis direction, and therefore, the rigidity of the skirt portion is lowered. Accordingly, thermal expansion is absorbed by deformation of the skirt portion even if the mask body is heated and thermally expanded by collision of electron beams. It is thus possible to reduce doming of the mask body which causes the main surface portion to expand toward the phosphor screen. As a result, landing drift of electron beams on the phosphor layers can be reduced and deterioration of color purity can be prevented.
Further, if openings or concave portions elongated in the long axis direction are provided at the non-aperture portion of the mask body, the difference in heat conductivity between the main surface portion and the non-aperture portion can be reduced, so that the temperature of the main surface portion is decreased while the temperature of the non-aperture portion is increased, in comparison with a conventional mask body. As a result, the temperature distribution of the entire mask body becomes uniform, and deterioration of color purity caused by landing drift of electron beams onto the phosphor layers can be prevented.
If each of the skirt portion and the non-aperture portion of the mask body is provided with openings or concave portions elongated in the long axis direction, the rigidity of the skirt portion is lowered and the difference in heat conductivity at the boundary portion between the main surface portion and the non-aperture portion can be reduced. Accordingly, it is possible to prevent more effectively deterioration of color purity caused by landing drift of electron beams on the phosphor layers.
Further, openings elongated in the long axis direction or concave portions having a bottom plate thickness smaller than the plate thickness of the mask body are formed in at least one of the skirt portion and the non-aperture portion, within a range of about xc2xc of a length of the mask body in the direction of the long axis of the mask body, with respect to a center of the range defined at a position distant from the short axis of the mask body by about ⅓ of the length of the mask body in the direction of the long axis. Therefore, localized doming is reduced at a portion where doming most easily occurs in case of a conventional mask body, and localized deterioration of color purity caused by landing drift of electron beams onto the phosphor layers can be effectively prevented.